Time-of-light flight type distance sensor

ABSTRACT

A lower cost range-finding image sensor based upon measurement of reflection time of light with reduced fabrication processes compared to standard CMOS manufacturing procedures. An oxide film is formed on a silicon substrate, and two photo-gate electrodes for charge-transfer are provided on the oxide film. Floating diffusion layers for taking charges out from a photodetector layer are provided at the ends of the oxide film, and on the outside thereof are provided a gate electrode for resetting and a diffusion layer for providing a reset voltage.

FIELD OF THE INVENTION

The present invention relates to a range-finding sensor, which measuresa delay time of a light pulse transmitted from a light source and thenreflected by a target object to be measured.

BACKGROUND-ART

The following references show related arts:

-   (1) Inventor: Cyrus Bamji, Assignee: Canesta Inc., “CMOS-Compatible    Three-dimensional image sensor”, U.S. Pat. No. 6,323,942 B1, Nov.    27, 2001;-   (2) R. Lange, P. Seitz, A. Biber, and S. Lauxtermann, “Demodulation    pixels in CCD and CMOS technologies for time-of-flight ranging”,    Proceedings of SPIE, Vol. 3965, pp. 177-188, 2000;-   (3) Ryohei Miyagawa, Takeo Kanade, “CCD-based range-finding sensor”,    IEEE Trans. Electron Devices, vol. 44, No. 10, pp. 1648-1652 (1997);-   (4) Range Imaging Device, Japanese Patent Application Laid-open No.    2001-281336; and-   (5) Charge Coupled Device, Japanese Patent Application Laid-open No.    2003-51988.

According to the method of (1), light pulses are transmitted, and thewaveform of received signal pulses is shaped, by detecting peaks of thereceived signal pulses so that a delay time can be digitally measuredusing high-speed pulses. Since the method of (1) cannot work wellwithout a sufficient light intensity that facilitates generation of apulse signal from the received signal, application fields of method of(1) are limited.

The architectures of the methods of (2) and (3) are similar to eachother. In the method of (2), in which CCD and CMOS are merged into asingle chip through an integrated fabrication process, with a highfrequency modulated light of 20 MHz, by utilizing a charge transfermechanism of the CCD, a characteristic such that the distribution ratioof charges into two nodes depends on the delay time of the modulatedlight, in synchronization with the modulated light, is utilized. Suchintegrated CCD-CMOS manufacturing procedure increases cost.

According to the method of (3), the structure of the CCD is utilized toalternately transfer charges, which are generated by a pulse-modulatedlight, to two nodes, and a characteristic of the resulting chargedistribution ratio depending on the delay time of the modulated light isutilized. Use of such a CCD requires a special fabrication process.Furthermore, while only a one-dimensional sensor (i.e., line sensor) isdisclosed, implementation of a two-dimensional sensor (area sensor)established only with CCDs may be difficult, considering that all of thepixels should be simultaneously driven at a high frequency.

According to the methods of (4) and (5), though no detailed structure isdisclosed, a structure in which charges generated by a photodiode aretransferred to floating diffusion layers via two transfer gates isemployed. However, incomplete transfer of the charges to two floatingdiffusion layers results in an insufficient performance. Therefore, acomplex fabrication process must be added to fabricate a CMOS structure,resulting in a high fabrication cost. Meanwhile, because integration ofa parallel-driving circuit for driving pixels is impossible in the CCDarchitecture by itself, an integrated CCD/CMOS manufacturing procedureis required for the CCD architecture. In conclusion, low cost does notgo with high performance.

SUMMARY OF THE INVENTION

It is preferable to manufacture a range-finding sensor with a maximumperformance at low cost, by adding the least possible number offabrication processes to a standard CMOS manufacturing procedure.

An object of the present invention is to provide a high performancerange-finding sensor, being manufactured by a standard CMOSmanufacturing procedure or by adding a simple fabrication process to thestandard CMOS manufacturing procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a pixel area in a TOF sensor;

FIG. 2 is a potential distribution diagram explaining an operation ofthe TOF sensor;

FIG. 3 is a block diagram of a TOF range-finding image sensor;

FIG. 4 is a timing chart showing behaviors of the circuit in FIG. 3;

FIG. 5 shows a structure of a pixel circuit having a sample and holdfunction;

FIG. 6 is a timing chart for a range-finding image sensor using thecircuit of FIG. 5, which includes timing for reading out backgroundlight;

FIG. 7 shows structures using n-type diffusion layers under a fieldoxide—FIG. 7( a) is a cross-sectional view, while FIG. 7( b) is a topview—in comparison with FIG. 1;

FIG. 8 shows a structure of a TOF pixel using a structure ofinterdigital electrodes;

FIG. 9 explains a mixture of signal charges due to leakage of light;

FIG. 10 shows a behavior of the TOF pixel using the structure of theinterdigital electrodes;

FIG. 11 shows a structure of a TOF sensor including three electrodes;

FIG. 12 explains a pixel circuit of FIG. 11;

FIG. 13 is a timing chart for control signals in the pixel circuit ofFIG. 11; and

FIG. 14 shows a pixel structure of a TOF sensor using a field oxideestablished by STI architecture.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

Complete transfer of charges is required to provide a time-of-flight(hereafter, called ‘TOF’) sensor. However, it has been consideredimpossible to achieve such complete transfer using a standard CMOSarchitecture. Accordingly, some fabrication processes have been added tothe standard CMOS manufacturing procedure. The present invention is toovercome such a problem by using a micro-fabrication technology and aspecial structure that can be fabricated using standard CMOSmanufacturing procedure.

FIG. 1 shows a structure of a pixel of a range-finding sensor accordingto the present invention. While a single pixel may be used by itself fora certain application field, arranging a plurality of pixelsone-dimensionally or two-dimensionally implements a range-findingsensor. FIG. 1( a) shows a cross-sectional view of the pixel of therange-finding sensor while FIG. 1( b) shows a plan view observed fromabove a surface of a semiconductor substrate—observed from a location inthe upper direction on the paper of the cross-sectional view. FIG. 2shows a potential distribution in and at the surface of thesemiconductor substrate, describing the behavior of the pixel of therange-finding sensor.

An insulator layer (3) is provided on the semiconductor substrate (20),and a couple of photo-gate electrodes (1 and 2) configured to transfercharges are provided on the photo-gate electrodes. At the ends of theinsulator layer, floating diffusion layers (5 and 6) configured toextract charges from a photodetector layer (4) are provided, and at theoutsides of the floating diffusion layers, gate electrodes for resettransistors and diffusion layers configured to supply reset-voltages tothe floating diffusion layers through the reset transistors areprovided. Hereinafter, an exemplary structure encompassing a p-typesilicon substrate as the semiconductor substrate (20), a field oxide asthe insulator layer (3), and polysilicon electrodes as respectivephoto-gate electrodes (1 and 2) will be explained.

As shown in FIG. 1, on the field oxide are provided two polysiliconelectrodes as close as possible to each other. Repetitive signal pulsesTX1 and TX2 having reverse phases to each other are applied to thepolysilicon electrodes, thereby operating the pixel. It is assumed thatthe polysilicon electrodes be thin enough for a received light with aspecific wavelength to pass through. Note that a compound called“silicide” made of metal and silicon is formed on the polysiliconelectrodes, and if the silicide does not have enough transmissivity fora received light to pass through, a process for preventing formation ofthe silicide or removing the silicide thereafter is needed. Furthermore,note that FIG. 1 is based on the assumption that a local oxidation ofsilicon (LOCOS) process is used for forming the field oxide.Illustration of the geometry of the LOCOS configuration is omitted inFIG. 2, because FIG. 2 is supposed to explain the operation principle ofthe pixel.

An aperture is provided only at the central region between tworespective polysilicon electrodes so that the other regions can beoptically shielded. It is preferable that the semiconductor layerbeneath the polysilicon electrodes be of a low concentration so that thestrongest possible fringing electric field can be established in thesemiconductor layer. Such low concentration region can be achieved by amanufacturing procedure such that the formation process of p-type wellis omitted against the standard manufacturing procedure, by which ap-type well and an n-type well is formed in a low concentration p-typesubstrate so as to implement a CMOS device. While, in general,channel-stop impurities may be introduced in the semiconductor layerbelow the field oxide so as to prevent the formation of inversion layerat the surface of the semiconductor layer, the introduction of thechannel-stop impurities should be prevented in the semiconductor layer.The prevention of the introduction of the channel-stop impurities may beachieved by changing the mask pattern used in the standard CMOSmanufacturing procedure; otherwise, an addition of a new mask and a newfabrication process are needed.

With such a structure, an exemplary potential distribution when applying0V and 3.3V, for example, to the TX2 and the TX1, respectively, is shownin FIG. 2. In this case, almost all of the charges generated by thelight coming from the aperture enter the floating diffusion layer (n⁺region) on the right side. This emanates from the potential distributionin FIG. 2, which is generated by applying such voltages to therespective gates, and an acceleration force towards the right side,which is established by the fringing electric field ascribable to thepotential distribution. Note that the voltages shown in FIG. 2 are mereexamples and the present invention is not limited the examples shown inFIG. 2.

Since the field oxide is relatively thick, the electric field applied tothe semiconductor surface will become relatively weaker and the fringingelectric field is accordingly expected to increment against thedecrement of the electric field at the semiconductor surface. Therefore,a scheme for preventing development of a potential hill at the gapbetween two polysilicon electrodes is required.

When n-type impurity atoms have been introduced into polysilicon gatesthat serve as the photo-gate electrodes, the work function for thesemiconductor substrate differs from that for the photo-gate electrodeside. Thus, when a high positive voltage (e.g., 3.3V) and 0V are appliedto the TX1 and the TX2, respectively, a certain size of a gap betweentwo photo-gate electrodes may develop a potential hill in the gap,preventing the charges in the TX2 side from moving to the TX1 side. Insuch a case, the potential hill can be removed by applying a negativevoltage (e.g., −1V) to the TX2. Application of such a negative voltageto the TX2 side relative to the substrate removes the potential hill,thereby electrons generated in the semiconductor directly beneath thepolysilicon electrode at the left side can be transferred to the rightside.

Because the smaller the gap between the polysilicon electrodes, the moredifficult it is for a potential hill to develop, the gap should befabricated with a smaller size using a micro-fabrication technology.

Note that the structure of the substrate may be implemented by means forforming a low concentration p-type silicon layer on a high concentrationp-type substrate such as an epitaxial growth technology, for example, orby means for forming the low concentration p-type silicon layer on ann-type substrate, using such as the epitaxial growth technology. Suchstructure of the substrate is effective in that the component ofcharges, the charges are generated in a deep region of the semiconductorlayer and are extracted slowly through diffusion, can be decreased so asto improve the range-finding resolution.

FIG. 3 is a block diagram of a range-finding image sensor made up oftwo-dimensionally arranged pixel circuits, one of which is shown inFIG. 1. FIG. 4 is a timing chart showing the behavior of therange-finding image sensor.

After high control signals R are applied to two floating diffusionlayers for every pixel so as to reset them, a pulsed light source isturn on, and repetitive pulses TX1 and TX2 are applied to all pixels insynchronization with the pulsed light source, thereby operating thepixels for a fixed time period. Afterwards, the pulsed light source isturned off, and the voltages of respective floating diffused layers arethen read out. This read-out is conducted by reading out everyhorizontal line to noise cancellation circuits for respective columns,canceling noise, and then scanning horizontally. Selection of eachhorizontal line is conducted by providing a control signal S to thepixel selection switch of the buffer amplifier in each pixel, resultingin signals from every horizontal line appearing in vertical signallines.

A noise cancellation circuit is a circuit configured to cancel thedifference between a signal level and a level after resetting thefloating diffusion layer, to reduce a fixed noise pattern and 1/f noisesgenerated by buffer amplifiers of respective pixels. Accordingly, asshown in FIG. 4, the noise cancellation circuit is configured to samplea signal level and a level after resetting, in synchronization withφ_(S) and φ_(R), respectively, and then to calculate the differencebetween the signal level and the level after resetting. Description ofthe noise cancellation circuit itself is omitted because the noisecancellation circuit has little relevance to the essence of the presentinvention.

Charges Q1 and Q2 accumulated in two floating diffusion layers, byapplying N times of the TX1 and TX2 so as to transfer N times theresulting charges generated by the applied light pulses into thefloating diffusion layers, are respectively represented by the followingequations:Q ₁ =N×I _(p)(T ₀ −T _(d))  (1)Q ₂ =N×I _(p) T _(d)  (2)where Ip denotes a photocurrent generated due to a received light, T₀denotes the width of a light pulse, and T_(d) denotes the delay time ofthe light pulse. The sum of the equations (1) and (2) is represented by:Q ₁ +Q ₂ =N×I _(p) T ₀  (3)

The delay time of the received light and a distance L to a target objectare calculated using the following equations according to the equations(1) and (2):T _(d)=(Q ₂/(Q ₁ +Q ₂))T ₀  (4)L=(c/2)(Q ₂/(Q ₁ +Q ₂))T ₀  (5)where c denotes the speed of light. Since the voltages of the floatingdiffusion layers are in proportion to Q1 and Q2, respectively, theequation (5) is represented by the following equation using outputsignal voltages V1 and V2:L=(c/2)(V ₂/(V ₁ +V ₂))T ₀  (6)

Since c and T₀ are known, the distance can be calculated from tworeceived output voltages using the equation (6).

Second Embodiment

The structure of FIG. 2 is easily influenced by a background light (ifit exists). That is, according to FIG. 2, even when reading out asignal, charges due to the background light are included in the signal,and furthermore, the times required for reading out the charges due tothe background light differ in respective horizontal lines. This makesit difficult to perform background light removal processing. To avoidthe difficulty in the background light removal processing, the read-outperiod needs to be sufficiently short relative to the TOF operatingtime, and thus high-speed read-out is required.

FIG. 5 shows a structure of a pixel circuit that is not influenced bythe background light when reading out a signal. FIG. 6 is a timing chartfor canceling the influence of the background light by reading out onlysignals due to the background light.

MOS transistors (9 and 10) are prepared on the right and the left side,respectively, so as to separate two floating diffusion layers from aphotodetector layer, and the gates of the MOS transistors (9 and 10) arecontrolled by a signal SH. The signal SH is set to a high level duringlight pulse reception, thereby connecting the two diffusion layers tothe photodetector layer. After the reception of the light pulse, the SHis then set to a low level, and thereby separating the two floatingdiffusion layers from the photodetector layer. At this time, the voltageof the floating diffused layer separated from the photodetector layer isread out, canceling the influence of the charges due to the backgroundlight incident on the photodetector layer during the read-out period.

The influence of the background light first needs to be cancelled duringreception period of a light pulse under the background light, and thenthe distance is to be calculated. Therefore, signals for respectivepixels generated by the background light are required. It is effectiveto use as the background signal the charges, which are accumulatedduring the read-out period of the signal generated by the light pulse.Accordingly, right after reading out the signals generated by the lightpulse from the pixels on a subject horizontal line, the floatingdiffusion layers of the pixels on the subject horizontal line are reset,the accumulation of the background signals is conducted for a fixed timeperiod, and then the same signal read-out operation of the backgroundsignals is conducted.

The accumulation period is set to be equal to the entire read-out periodof the signals from all of the pixels. To achieve a uniform detectionsensitivity, the SH is set to a high level. FIG. 6 shows a timing chartfor reading out signals from one of horizontal lines. The reading outoperation shown in FIG. 6 is conducted for signals from all horizontallines. The background light accumulation period for each of thehorizontal lines is the same. When the accumulation period during lightpulse irradiation (T_(L)) differs from the all-pixel read-out period(T_(LR)), consideration of the difference of periods is needed whencanceling the background light.

The above-mentioned cancellation of the background light is no problemwhen a target object is at a standstill. Measurement with a movingtarget object, however, includes an error because the background signalsare taken out for the cancellation process at a different timing duringthe light pulse irradiation. To decrease the error, a cycle perioddefined by (T_(L)+T_(LR)+T_(BR)) shown in FIG. 6 is made shorter.

Furthermore, to increase the amount of signals, the operation of thecycle shown in FIG. 6 is repeated, and the resulting signals areintegrated in an external circuit. Alternatively, calculation ofdistance is conducted in every cycle and is repeated multiple times, andan average of the resulting calculated distances is then calculated,thereby simplifying that operation. Moreover, such an averagecalculation is effective for improving the range-finding resolution.

Note that since the sum of two pixel outputs corresponds to intensityimage information of a target object, intensity image information andrange image information can be obtained at the same time according tothe present invention. The methodology of summing the two pixel outputsfacilitates applications such as three-dimensionally displaying anintensity image using a combination of the intensity image informationand the range image information.

Third Embodiment

In a structure shown in FIG. 1, if a fabrication process of formingother n-type diffusion layers under the field oxide is available, astructure of connecting the photodetector layer to the floatingdiffusion layers, which serve as the sources (or drains) of the MOStransistors via the n-type diffusion layers (13 and 14) is possible, asshown in FIG. 7. When a wide (along the depth direction to the plane ofthe paper) gate width of a photodetector layer is designed so as toensure a sufficiently large optical detection area, the greater the areaof the high-concentration source or drain region of the MOS transistor,the more the dark current increases and the more the capacitance alsoincreases, decreasing the voltage sensitivity. Therefore, as shown inFIG. 7, the n-type diffusion layers are provided so that the n-typediffusion layers can capture electrons generated in the photodetectorlayer.

Note that an n-type well layer may be used as the n-type diffusion layerbeing formed under the field oxide.

Fourth Embodiment

FIG. 8 shows a layout of a TOF pixel structure encompassing twocomb-shaped photo-gate electrodes arranged interdigitally. In theFigure, reference numerals 31 denote photo-gate electrodes; referencenumeral 32 denotes a boundary line indicating that the inside of theboundary line is an aperture into which light enters and the outside isa optically shielded area; reference numerals 33 denote n-type diffusionlayers buried under the field oxide; reference numeral 34 denotes aboundary line indicating that the outside of the boundary line is ap-type well region and the inside is a p-type semiconductor substrate;reference numerals 35 denote source or drain regions (n⁺ diffusionlayers) of MOS transistors; and reference numerals 36 denote diffusionlayers or metal interconnects configured to be connected with thephoto-gate electrodes.

Such an interdigital configuration of the comb-shaped electrodes allowsthe TOF range-finding sensor to have a high charge-separationcapability, when dividing/delivering the charges into twocharge-detectors in accordance with the amount of pulse delay, therebyproviding a higher range-finding resolution.

With such a simple structure made from two rectangular photo-gateelectrodes facing each other, since a light may leak into a part of thephoto shield so as to generate signal charge (E) as shown in FIG. 9,which is fundamentally supposed to be transferred into thecharge-detector on the right side; in reality, however, the generatesignal charge (E) will be transferred to the charge-detector on the leftside. The mal-transfer charge causes a lower separation performancebetween two signals, lowering the range-finding resolution. To avoid theproblem associated with charge-separation capability, the area of theaperture for light to pass through should be decreased so that the lightcan irradiate only the central region of the pixel. However, the loweraperture ratio lowers the sensitivity.

FIG. 10 shows a surface voltage distribution along the cross-sectiontaken on line A-A in FIG. 8, in the interdigital configuration of thecomb-shaped electrodes. With such an interdigital configuration of thecomb-shaped electrodes, since the charge-transfer direction isorthogonal to the direction connecting between the two charge-detectors,a high separation capability for charges flowing into twocharge-detectors can be achieved, even if the teeth of the interdigitalelectrodes are lengthened so as to establish a sufficiently largeoptical aperture. Furthermore, since the width of teeth in thecomb-shaped electrodes can be made narrow enough, effective utilizationof fringing electric fields is possible, increasing the resolution.

Fifth Embodiment

Furthermore, if the width of the polysilicon electrode is too large, thefringing electric field is made weaker, and thus charge-transfer may bedriven insufficiently. Accordingly, three electrodes are prepared asshown in FIG. 11, and a method of providing control signals to threeelectrodes at timings as shown in FIG. 13 may be used. Behaviors withthree electrodes are shown in FIG. 12. For transferring the signal tothe floating diffusion layer on the right side, high positive voltagesare applied to the TX1 and TX3, respectively, TX3 is a control voltagefor the central electrode, and 0V is applied to the TX2 (FIG. 12( a)).As a result, the charges stored beneath the TX1 electrode aretransferred to the floating diffusion layer on the right side whilecharges under the TX3 electrode are temporarily stored beneath the TX3electrode.

Just before changing the charge-transfer direction to the left side, theTX3 is changed to be 0V for a short period, thereby transferring thecharges beneath the TX3 electrode to the right side (FIG. 12( b)).Afterwards, the TX1 is changed to be 0V while positive voltages areapplied to the TX2 and the TX3, respectively, thereby changing thecharge-transfer direction to the left side (FIG. 12( c)). Not shown inFIG. 13, the TX3 is changed to be 0V for a short period just beforechanging the charge-transfer direction to the right side, therebytransferring the charges beneath the TX3 electrode to the left side.These operations are repeated. Through these operations, sufficientfringing electric fields are utilized to transfer charges at a highspeed while ensuring a sufficient optical detection area.

To generate a two-dimensional image with a single photo-detector layer,an optical beam scanning means such as a rotating polygon mirror or avibrating mirror, which scans incident beams from a two-dimensionalsurface, may be employed. Alternatively, a combination of arange-finding sensor made from linearly arranged photo-detector layersand a similar optical beam scanning means may also provide atwo-dimensional image.

INDUSTRIAL APPLICABILITY

The diagrams of the structures demonstrated thus far assume that thestructure of the actual field oxide is implemented by a local oxidationof silicon (LOCOS) method. Alternatively, the field oxide may beimplemented by a shallow trench isolation (STI) method. FIG. 14 shows anexemplary structure that is the same as that of FIG. 7 and ismanufactured by a CMOS image sensor fabrication process with the STI.

In the same manner, the structures shown in FIGS. 1, 5, and 8 can besubstituted by the STI structure.

The structures and ideas according to the present invention can beapplied to all structures used in CMOS LSIs or CMOS image sensors, inaddition to these cases without losing generality. That is, a structureembracing two adjacent light transmissive electrodes (generally made ofpolysilicon), which serve as optical-receiving portions formed on afield oxide, and two n-type-regions serving as two floating diffusionlayers, respectively, shall be employed so that charges can betransferred to the floating diffusion layers.

1. A time-of-flight range-finding sensor for range-finding by reading asignal, which depends on a delay time of repetitive light plusestransmitted from a light source and then reflected by a target object tobe measured, the time-of-flight range-finding sensor comprising: ap-type semiconductor substrate having a p-type photodetector layer and ap-type well formed at a surface of the semiconductor substrate so as toencircle the photodetector layer in a plan view, the p-type well havinga higher concentration than the semiconductor substrate; an insulatorlayer formed on the photodetector layer; two conductive photo-gateelectrodes provided on the insulator layer above the photodetectorlayer, adjacently disposed so as to define a gap between the twophoto-gate electrodes, and being transparent for a wavelength of a lightreflected by the target object; and two MOS transistors formed in thep-type well so as to sandwich the photo-gate electrodes, each of the MOStransistors having an n-type floating diffusion layer serving as asource region, which is disposed at a boundary between the photodetectorlayer and the p-type well and configured to extract charges from thephotodetector layer, wherein a uniform optical path exists along thefull-width of the gap.
 2. The time-of-flight range-finding sensoraccording to claim 1, wherein each of two photo-gate electrodes has acomb-shaped geometry having a plurality of projections in a plan view,the projections of one of the photo-gate electrodes are insertedinterdigitally between the projections of the other photo-gateelectrode.
 3. The time-of-flight range-finding sensor according to claim1, further comprising signal-extraction MOS transistors, wherein each ofthe two MOS transistors further comprises: a second floating diffusionlayer serving as a drain, being connected to a gate electrode of one ofthe signal-extraction MOS transistors; and a gate electrode to beapplied with gate voltage, being controlled so as to electricallyseparate the first floating diffusion layer from the second floatingdiffusion layer configured to allow storage of an analog signal.
 4. Thetime-of-flight range-finding sensor according to claim 1, wherein theinsulator layer utilizes a field oxide being formed in a manufacturingprocedure of a CMOS integrated circuit.
 5. The time-of-flightrange-finding sensor according to claim 1, further comprising twodiffusion layers provided under the insulator layer, between thephotodetector layer and the n-type floating diffusion layers, beingdoped with impurity atoms having the same polarity as impurity atoms ofthe n-type floating diffusion layers.
 6. The time-of-flightrange-finding sensor according to claim 1, wherein the photo-gateelectrodes are made of polysilicon, which is the same material as thegate electrode of a MOS transistor in a CMOS integrated circuit, orpolysilicon and silicide formed on the polysilicon, the silicide beingtreated so as to increase optical transmissivity.
 7. The time-of-flightrange-finding sensor according to claim 1, wherein the photodetectorlayer utilizes the p-type semiconductor substrate, such that both ap-type well and an n-type well are not formed in the photodetectorlayer.
 8. The time-of-flight range-finding sensor according to claim 1,wherein the photodetector layer utilizes a low concentration n-typesemiconductor substrate, such that both a p-type well and an n-type wellare not formed in the photodetector layer.
 9. The time-of-flightrange-finding sensor according to claim 1, wherein a plurality of unitstructures, each of which comprising the photo-gate electrodes, thephotodetector layer, and the n-type floating diffusion layers, arearranged one-dimensionally or two-dimensionally so as to generate animage representing a range distribution.
 10. The time-of-flightrange-finding sensor according to claim 1, further comprising a lightbeam scanner configured to generate incident beams into therange-finding sensor from a two-dimensional plane so as to generate animage representing a range distribution.
 11. The time-of-flightrange-finding sensor according to claim 1, wherein range information isobtained from the ratio of two signals taken out respectively from thephoto-gate electrodes, while intensity information is obtained from thesum of the two signals.